1. Field of the Invention
The present invention relates to a stepping motor. The invention particularly relates to a hybrid stepping motor which is high in torque and less in vibrations, and to a rotor of such a hybrid stepping motor.
2. Description of the Related Art
Stepping motors of the permanent magnet type are used widely in positioning control, velocity control, or the like, in the field of office automation, factory automation, etc. Further, a synchronous motor in which such a stepping is arranged to be driven by a sinusoidal wave voltage is also used for constant velocity control, and so on.
A stepping motor using a permanent magnet in a rotor or a stator has features that the motor is high efficiency, superior in damping characteristic, etc., in comparison with a stepping motor of the variable reluctance type in which no permanent magnet is used because a permanent magnet is used in a part of a magnetic field in the stepping motor of the permanent magnet type. In such a stepping motor of the permanent magnet type, however, there is a problem that if this stepping motor is subjected to microstep driving in which driving currents of two phases are changed stepwise and relative to each other, the angular accuracy is poor and vibrations are much.
FIG. 9 is a section showing the structure of an inner-rotor type hybrid stepping motor which has been used conventionally; and FIG. 10 is a section along an A--A line in FIG. 9.
As shown in FIG. 9, in an inner-rotor type hybrid stepping motor, a stator 10 in which a winding is wound on a stator iron core is held by end brackets 9 and 11 therebetween and a rotor 12 in which two rotor yokes 12-1 and 12-2 and a permanent magnet 5 sandwiched between the two rotor yokes 12-1 and 12-2 are integrally fixedly mounted on a rotorshaft 6 is disposed inside the stator 10 so as to face the latter through an air gap therebetween and rotatably supported by a pair of bearings 7 and 7 provided on the end brackets 9 and 11 respectively. Each of the rotor yokes 12-1 and 12-2 has pole teeth 8 provided on its radially outer circumference. As shown in FIG. 10 which is a section along an A--A line in FIG. 9, the stator 10 has a plurality of (eight in the illustrated example) magnetic poles 1p through 8p inwardly radially provided on the inside of a ring-like yoke 10-Y. Each of the magnetic poles 1p through 8p has a plurality of pole teeth provided on the radially inward end thereof. Windings 1c and 2c are wound alternately on the magnetic poles 1p through 8p. In each of the rotor yokes 12-1 and 12-2, the pole teeth 8 are provided at equal pitches on the outer circumference thereof so as to be in opposition to the pole teeth of the stator 10 through an air gap therebetween. In the example of the stepping motor illustrated in FIG. 10, since the number Nr of the pole teeth provided on the outer circumference is 50 and the stator 10 has two-phase windings, the step angle .theta..sub.s is 1.8 degrees.
That is, since a phase difference is generated between retention torque Te which is generated when DC currents are made to flow in the the stator windings 1c and 2c to excite the stator windings and so-called cogging or detent torque Td which is generated in a non-excitation state in which no current is made to flow in the stator windings 1c and 2c, the composite torque Tf has a waveform which is distorted compared with a sinusoidal waveform as shown in FIG. 11 by way of example so that the angular accuracy becomes poor in the case of excitation by microstep driving and vibration becomes large even in the case of driving with sinusoidal currents.
Further, when the output of a motor is to be made high without changing the outer size of the motor, a measure in which two or more rotor units are integrally coupled to a single rotary shaft of the motor to thereby increase the output is widely used. In a hybrid stepping motor having a rotor in which a permanent magnet magnetized in the axial direction of the rotor is held between two rotor yokes each provided with a plurality of pole teeth on its outer circumference, in the case where it is intended to increase the output torque by increasing the axial thickness of the stator, the quantity of generated magnetic flux has a limit even if the thickness of the permanent magnet is increased, and, therefore, a measure in which two or more rotor sets coupled on the same rotor shaft are employed with the stator increased to increase the output torque is used.
FIG. 12 is a section showing the structure of a hybrid stepping motor in which in order to increase the output torque, two rotor sets 12a and 12b are coupled on one and the same rotary shaft 6 to constitute a rotor 12 which is arranged to face a stator 10, FIG. 13 is a detailed section of the rotor portion of FIG. 12, and FIG. 14 is a typical side view showing the arrangement of the pole teeth of the rotor of FIG. 13. The section along the line A--A in FIG. 12 is the same as that shown in FIG. 10.
In FIG. 12, the stator 10 having windings wound thereon is held between end brackets 9 and 11 and the rotor 12 is rotatably supported by a pair of bearings 7 and 7 provided on the end brackets 9 and 11 respectively.
As seen in FIG. 10, the stator 10 of FIG. 12 has a plurality (eight in the illustrated example) of radially provided magnetic poles each of which has a winding wound thereon and has a plurality of pole teeth formed on the radially inward end thereof. The rotor set 12a is constituted by rotor yokes 12-1 and 12-2 and the rotor set 12b is constituted by rotor yokes 12-3 and 12-3, and each of the rotor yokes 12-1, 12-2, 12-3 and 12-4 has pole teeth 8 provided on the outer circumference thereof so as to face the pole teeth of the stator 10.
As shown in FIG. 13 showing the detailed section of the rotor 12, the rotor yokes 12-1 and 12-2 having the same configuration with each other are arranged so as to sandwich a permanent magnet 5 therebetween under the condition that the arrangement of the pole teeth of the rotor yoke 12-1 is displaced in angular position by 1/2 pitch from the arrangement of the pole teeth of the rotor yoke 12-2 to thereby constitute the first rotor set 12a, and the rotor yokes 12-3 and 12-4 having the same configuration with each other are arranged so as to sandwich another permanent magnet 5 therebetween under the condition that the arrangement of the pole teeth of the rotor yoke 12-3 is displaced in angular position by 1/2 pitch from the arrangement of the pole teeth of the rotor yoke 12-4 to thereby constitute the second rotor set 12b. The two rotor sets 12a and 12b are fixed on the rotor shaft 6 separately from each other in the axial direction through a spacer 13 therebetween in a manner so that the respective arrangements of the pole teeth of the rotor yokes 12-1 and 12-3 of the respective first and second rotor yokes 12a and 12b are made coincident with each other in angular position and the respective arrangements of the pole teeth of the rotor yokes 12-2 and 12-4 of the respective first and second rotor yokes 12a and 12b are made coincident with each other in angular position.
FIG. 15 shows an example of a stepping or synchronous motor having a rotor constituted by a cylindrical permanent magnet in which in order to increase the output torque, two stator sets 10a' and 10B' having the same configuration with each other are provided and two rotor sets 12A' and 12b' having the same configuration with each other and fixed on one and the same rotor shaft are assembled into one body.
FIG. 16 is a detailed view of the rotor arrangement of FIG. 15, in which the two rotor sets 12a' and 12b' are integrally fixed on the rotor shaft in a manner so that the two rotor sets 12a' and 12b' are axially coincident in polarity with each other.
In such a configuration in which two rotor sets are provided on one and the same rotor shaft as shown in FIGS. 13 trough 16, the torque is increased but the increase of the torque may cause a problem that vibrations and noises are also increased. In a stepping motor, there has been a problem that vibrations and noises are large in comparison with an induction motor or the like and rotation is not smooth by an influence of the cogging torque of the rotor at the time of microstep driving, because the rotor rotates with incremental movement even in the case of using only one rotor set or only one permanent magnet.